The present invention generally relates to the press-molding of information carrier discs such as, for example, videodiscs, digital audiodiscs, and disc memories, and more particularly to a method and an apparatus for making such information carrier discs.
A fairly long time has passed since the optical disc recording/reproducing system has been made available in the market. Information carrier discs used in association with the optical disc recording and/or reproducing system are generally made by molding a synthetic resin. In general, photographic records are manufactured by the use of a plastics extruding machine designed to soften, knead and press-mold plastic material to form disc-shaped records.
However, when it comes to the information carrier discs for use with the optical disc recording/reproducing system (which carrier discs are hereinafter referred to as "optical carrier discs"), an injection molding technique is generally used to make them because no inclusion of foreign matter is permitted and because they must have a precise flatness and a precise roundness in order for an optical pick-up to coordinate with them. FIGS. 1 and 2 of the accompanying drawings illustrate an example of the conventionally used injection molding machines and a portion of the molding cavity in the injection molding machine, respectively, reference to which will now be made for the discussion of the prior art method for the manufacture of the optical carrier discs.
Referring first to FIG. 1, the prior art injection molding machine comprises a hopper 1 for accommodating and drying a synthetic material, a plasticizing cylinder 2 having a screw conveyor built therein, a screw drive 3 for driving the screw conveyor axially of and about the longitudinal axis of the cylinder 2, heaters 4 for heating the cylinder 2, a fixed platen 5, a movable platen 6, a plurality of parallel tie bars 7, a hydraulic cylinder 8 for driving the movable platen 6 in a direction close towards and away from the fixed platen 5, a fixed mold 10 carried by the fixed platen 5, and a movable mold 11 carried by the movable platen 6 and cooperable with the fixed mold 10 to define a molding cavity 15 (FIG. 2), complemental in shape and size to the desired carrier disc, when the movable platen 6 is driven close towards the fixed platen 10 with the molds 10 and 11 clamped together. FIG. 1 shows how the synthetic material is injected and then molded in the molding cavity 15, it being noted that the fixed mold 10 has a sprue 13 defined therein in communication with the cylinder 2 and also has a molding surface 12 formed with signal grooves.
The machine shown is so designed that the synthetic material within the hopper 1 can be fed forward by the action of the screw conveyor within the cylinder 2 while being simultaneously plasticized and then injected into the molding cavity 15 through the sprue 13 and then through a gate 14 by the push of the screw conveyor to form a molded article 16 which is subsequently allowed to solidity.
The conventional manufacture of the carrier discs by the use of the injection molding machine has the following problems because the synthetic material is necessarily plasticized in order to render it to flow.
(a) Flow markings tend to appear on one or both of the opposite surfaces of the carrier disc, which often constitute a cause of generation of erroneous signals from the pick-up.
(b) Molecules are oriented in a direction generally parallel to the direction of flow of the plasticized synthetic material, constituting a cause of birefringence of light, and accordingly, an error tends to occur during the detection of a component of the laser beam reflected from the surface or surfaces of the carrier disc.
(c) Since the plasticized synthetic material is injected into the narrow molding cavity in a short time, considerably increased pressure as well as increased clamping force are required, resulting in the necessity for using an injection molding machine which is bulky in size and, hence, expensive to manufacture.